The SureFire DG Switch gets along with... pretty much no holster out there. I have ran into issues with every holster I've tried.
There are a couple ways to skin this cat, the obvious one is to break out the Dremel tool, some jewelers files, heat gun, sandpaper, etc. This is a must, otherwise your holster will eat your DG Swtich for lunch. And while SureFire has a great warranty, nobody wants to continuously send a product back.
In addition to this, I feel it beneficial to beef up the switch. I swear I read a article here a long while back about reinforcing the DG Switch. I can't seem to find it. But, I did recently post a video tutorial on how to do it: https://kitbadger.com/dg-hack/
Ultimately you put a piece of masking tape (colored ideally) along the bottom of your DG Switch. Then you put a piece of 1/2" heat shrink over the taped switch and apply heat. In doing so, you have given yourself a sacrificial piece of heat shrink to slowly get abused. And the colored masking tape allows you to immediately see, upon visual inspection, if your sacrificial layer has been worn through. At which time you can replace it. *Mind you, you should still modify your holster.*